tig: fix zsh completion

The ZSH completion script of `tig` basically imports the bash completion
script and uses it as completion approach. Unfortunately the script
takes several assumptions about the directory structure using
`$funcsourcetrace`[1] that don't apply on NixOS.

The easiest workaround is t opatch the completion script and import the
bash completion script from `$out`.

[1] http://zsh.sourceforge.net/Doc/Release/Zsh-Modules.html#index-funcsourcetrace

pkgs/applications/version-management/git-and-tools/tig/default.nix

@@ -31,6 +31,10 @@ stdenv.mkDerivation rec {
   installPhase = ''
     make install
     make install-doc
+
+    substituteInPlace contrib/tig-completion.zsh \
+      --replace 'e=$(dirname ''${funcsourcetrace[1]%:*})/tig-completion.bash' "e=$out/etc/bash_completion.d/tig-completion.bash"
+
     install -D contrib/tig-completion.bash $out/etc/bash_completion.d/tig-completion.bash
     install -D contrib/tig-completion.zsh $out/share/zsh/site-functions/_tig
     cp contrib/vim.tigrc $out/etc/